beszel: fix testing compilation and darwin issues

[arunoruto]

The testing tag introduces a problem where the internal/hub/systems/systems_test_helpers.go file is being compiled, which should be a moc file for testing, instead of the actual file systems_production.go. By removing the testing tag and just adding it to the checkFlags variable, this should be solved.

Resolves: #510220

Darwin tries to run the GPU tests, which fails, since most of the utilities needed are not available for darwin systems (nvidia and amd). We skip those tests just for the darwin systems.
Darwin has also a weird behaviour where a racing condition is happening. To solve this, we need to introduce a small defer statement in the function which causing the racing condition.

Resolves: #512864

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

[BonusPlay]

A friend of mine has suggested following things instead of patching the code:

1. __darwinAllowLocalNetworking = true;
2. Add following tests to the exclusion:

skippedTests = [
      "TestCollectorStartHelpers"
      "TestConfigSyncWithTokens"
      "TestApiRoutesAuthentication"
      "TestNewGPUManager"
    ]

[arunoruto]

1. __darwinAllowLocalNetworking = true;

I tried it but I am still getting:

beszel> panic: runtime error: invalid memory address or nil pointer dereference
beszel> [signal SIGSEGV: segmentation violation code=0x2 addr=0xb0 pc=0x1007db5b0]
beszel>
beszel> goroutine 39250 [running]:
beszel> github.com/pocketbase/pocketbase/core.(*BaseApp).RecordQuery(0x4e4027620708, {0x101dbdee0?, 0x4e40278c6908?})
beszel>         /nix/var/nix/builds/nix-11741-2322230898/source/vendor/github.com/pocketbase/pocketbase/core/record_query.go:38 +0x70
beszel> github.com/pocketbase/pocketbase/core.(*BaseApp).FindRecordById(0x4e4027620708, {0x101c335e0?, 0x101df26b0?}, {0x4e40276241f0, 0xf}, {0x0, 0x0, 0x1009662e8?})
beszel>         /nix/var/nix/builds/nix-11741-2322230898/source/vendor/github.com/pocketbase/pocketbase/core/record_query.go:223 +0x78
beszel> github.com/henrygd/beszel/internal/hub/systems.(*System).getRecord(0x4e4027998a80, {0x101e22aa8?, 0x4e4027996000?})
beszel>         /nix/var/nix/builds/nix-11741-2322230898/source/internal/hub/systems/system.go:348 +0x50
beszel> github.com/henrygd/beszel/internal/hub/systems.(*System).setDown(0x4e4027998a80, {0x101df9e00, 0x4e4027573ae0})
beszel>         /nix/var/nix/builds/nix-11741-2322230898/source/internal/hub/systems/system.go:373 +0xc4
beszel> github.com/henrygd/beszel/internal/hub/systems.(*System).StartUpdater(0x4e4027998a80)
beszel>         /nix/var/nix/builds/nix-11741-2322230898/source/internal/hub/systems/system.go:89 +0xc4
beszel> created by github.com/henrygd/beszel/internal/hub/systems.(*SystemManager).AddSystem in goroutine 38496
beszel>         /nix/var/nix/builds/nix-11741-2322230898/source/internal/hub/systems/system_manager.go:255 +0x17c
beszel> FAIL    github.com/henrygd/beszel/internal/alerts       68.499s
beszel> FAIL

But it seems like the problem does appear upstream and should be fixed with this PR: henrygd/beszel#1951

[BonusPlay]

I guess we could merge this one (I'd just add comment on the patchPhase with link to the upstream PR)